1. Field of the Invention
The present invention relates to a tandem-type master cylinder having two pressure chambers, and more particularly to a master cylinder in which a first pressure chamber is formed between primary and secondary pistons in a sleeve which is inserted and disposed in the cylinder main body.
2. Prior Art
A tandem-type master cylinder having two pressure chambers is one of the options for use with the automobile brake systems. There are two types of twin-pressure-chambered tandem-type master cylinders: in one type, cup seals are provided on the pistons, while in the other, they are provided on the cylinder main body. The latter is advantageous in design of a compact master cylinder, in which cup seals are designed to be held by a sleeve inserted in the cylinder main body for mounting therein.
Japanese Patent Public Disclosure No. 16952/1983 which corresponds to U.S. Pat. No. 4,510,752 discloses a master cylinder of this type. In this master cylinder, a primary cup and secondary cup are held between the internal wall of the cylinder main body and either end of a sleeve inserted in the same, respectively. Moreover, a first pressure chamber is formed between two pistons in the sleeve, and a secondary pressure chamber between the distal end of the cylinder main body and one of the two pistons, a secondary piston. In addition, two inlet ports and two outlet ports are provided on the cylinder main body through the circumferential wall thereof in such a manner as to connect the two pressure chambers, respectively, to a brake fluid reservoir and to wheel cylinders, and a through-hole is provided on the sleeve, communicating with one of the outlet ports on one side and with the first pressure chamber on the other side. This through-hole is elongated longtudinally of the sleeve lest it should be completely closed by the piston during the reciprocating movement of the piston. Return springs for returning the pistons to their normal positions are designed to be lodged in holes formed in the respective pistons to accommodate them, and this also serves to make the master cylinder more compact.
A master cylinder of this type, however, tends to offer a lower degree of mechanical strength of the sleeve due to the presence of the elongated through-hole formed in the sleeve. Moreover, although it is ideal to locate the elongated through-hole at a position facing the top portion of the circumferential wall of the cylinder main body in order to prevent air from remaining in the pressure chambers at the time when filing the brake system with brake fluid, it cannot be completely assured with such a prior art master cylinder that the sleeve is so positioned relative to the cylinder main body that the elongated through-hole will be located at the ideal position when the former is inserted in the latter. It is possible that the elongated through-hole may be located at a position facing the side of the circumferential wall of the cylinder main body, in which case air tends to remain in the pressure chambers.